If you use the "unused cycles" - when you work and the computer is idle, this may be OK since you use the electrical power anyhow.
I do not think it pays to have computers running 24/7 ... unless you have a 6 or 8-core computer (as I understand it GPUs are not used). That would probably yield 4 - 6 USD per day and computer. If the computer draws 150 W per hour (which is reasonable) that is 3.6kWh/24h at a cost of .12 USD/kWh that would cost less than 0.50 USD/day. the profit would be - optimistically - 3.50 USD/day, 105 USD/month.
I would jump at this if it were cash. Since I - in Sweden - have to pay VAT (=sales tax) of 25% on everything which I would like to have (e.g. HDD, CPUs, GPUs, RAM, mobos) plus customs costs (some 10-12% of the value), the surcharge would be 30-35 USD for each 100USD earned. If they would be located in the European Union, I would also consider it - no customs costs, no VAT.